1. Field of the Invention
The present invention relates to a low melting glass, a sealing composition and a sealing paste, and more particularly to those which contain substantially no lead component and are used for manufacturing of a flat panel display such as a plasma display panel (hereinafter referred to as the PDP).
2. Description of the Related Art
Conventionally, a sealing composition used to seal the periphery of the PDP was generally a lead-based material consisting of lead-based glass powder of PbO—SiO2—B2O3 or the like and a filler such as refractory ceramics powder. But, in these years, it is demanded to develop a composition, which does not contain a hazardous component such as a lead component for environmental reasons and can seal at a low temperature.
As a low melting glass not containing a lead component and the like, there are known phosphate glass, borosilicate glass, alkali silicate glass and bismuth glass, among which the bismuth glass attracts attention because it can be baked at a low temperature and is excellent in chemical endurance.
The bismuth glass which has been developed by now has a coefficient of thermal expansion of 90 to 110×10−7/° C. which is different from a coefficient of thermal expansion (70 to 80×10−7/° C.) of glass which is used for sealing of a display and its accessories (such as a flat backlight). In order to match them, the bismuth glass is blended with a low-expansion refractory ceramics filler.
But, if an amount of ZnO contained in the low melting glass composition using the bismuth glass is little, a coefficient of thermal expansion becomes 110×10−7/° C. or more. Meanwhile, if a large amount of refractory ceramics filler is blended in order to reduce the coefficient of thermal expansion, the material has an increased viscosity, and sealing is disabled at less than 500° C. (see Japanese Patent Laid-Open Application No. Hei 9-278483). Conversely, a low melting glass containing a large amount of ZnO is also developed but has a disadvantage that its chemical endurance is degraded because it contains a large amount of Bi2O3 but does not contain Al2O3 (see Japanese Patent Laid-Open Application No. Hei 10-139478).
Meanwhile, when the PDP is produced, a sealing material which has a sealing composition in a paste form is baked at a sealing temperature of 500° C. or less (preferably 480° C. or less) in order to suppress excessive flow of a sealing portion, generation of foams and also a thermal damage to other members such as ribs, phosphors, electrodes. But, because these members are also produced by the same baking treatment as the sealing of the sealing material, there were problems that the process became long when they were baked separately and the manufacturing cost became high.
To remedy the problems, it has come to shorten the process by baking the plural members at the same time in these years, and the sealing material is also degreased and pre-baked simultaneously in combination with the other members. In other words, the sealing temperature of the other members and the pre-baking temperature of the sealing material have become the same. Therefore, the baking temperature of the sealing material is required to be lower than the baking temperature of the other members such as ribs, phosphors, electrodes and the like. As a result, as the sealing material, there is a demand for a low melting glass of which fluidity is not impaired even if it is heated to a pre-baking temperature higher than the sealing temperature and can make sealing.
A conventional lead-based glass had a wide sealable temperature range (in which the low melting glass can seal without crystallizing when it is used as amorphous glass), satisfying the above-described demanded characteristics. But, the bismuth glass could seal at a low temperature but had a narrow sealable temperature range. Therefore, if the pre-baking temperature exceeds the sealable temperature range, there are problems that the bismuth glass becomes more likely to generate crystal because of Bi2O3 as well as B2O3 and ZnO contained in its glass components, and sealing cannot be made at the sealing temperature when baking.
The bismuth glass has superior fluidity at 500° C. or more to other low melting glasses. Therefore, when it is pre-baked at 500° C. or more, the sealed portion of the sealing material flows excessively, and a recess is produced at the center. This recess forms a cavity and remains as a remaining foam after sealing, and the adhered surface is deformed, causing a problem that air tightness cannot be maintained by the sealing material.
Besides, for mass production, it is desirable to perform melting by a continuous melting furnace which continuously performs a series of steps from supplying of materials to forming. To obtain homogeneous glass in the same way as the lead-based glass, it is generally necessary to heat the bismuth glass to 1000° C. or more for melting. But, when refractory bricks are used as the furnace material, elutes (e.g., alkali metal oxide, zirconia, etc.) from the surfaces of bricks increase because the bismuth glass is highly erosive. And, the elutes make a crystal nucleus of glass to cause a problem that the glass is crystallized when pre-baking. Because this bismuth glass has very low viscosity when melting and small surface tension, there are problems that the glass might leak through joints of the bricks which were enlarged by erosion when operating, and the glass cannot be melted stably for a long period of time.
As described above, as the manufacturing process is shortened, the sealing material is heated to a temperature exceeding the sealing temperature in the pre-baking step. Even after the same pre-baking, the conventional lead-based material could seal by baking at a temperature lower than the pre-baking temperature.
But, the bismuth glass developed heretofore could not seal at a temperature lower than the pre-baking temperature in the manufacturing of particularly a flat panel display such as the PDP. Even if sealing could be made, the glass which could seal finely was not found because sealing strength was low or foams remaining after sealing increased.
The melting furnace having platinum or an alloy of platinum-rhodium coated on its surface was not satisfactory as a continuous melting furnace capable of mass-producing stably for a long period because platinum or the like was heavily deteriorated (erosion or cracks) when the bismuth glass is used.
Accordingly, the present invention has been made to remedy the problems of the above-described conventional sealing composition, and provides a low melting glass formed of bismuth glass, which is not crystallized even if pre-baked at a sealing temperature or more, can seal at a pre-baking temperature or less and can be melted stably for a long period while suppressing platinum, platinum-rhodium or the like from deteriorating, a sealing composition using the above glass and a sealing paste. Simple indication “%”, in the specification means “mass %”.